Slugging is basically structured hitch hiking. There are specific locations where riders wait for drivers to pull up and take you to certain destinations. Some suburban slug lots feed in to multiple destinations in the District, while others only go to one. Riders get free rides, and drivers get enough passengers to qualify to drive on the MUCH faster HOV-3 lanes.
The spot I picked is not far from my house and goes exclusively to the Pentagon. My plan was to catch a shuttle from the Pentagon to my actual office, which is in Rosslyn. It turns out the driver I picked was ultimately heading to Georgetown, which is right near Rosslyn, so she brought me straight there.
And while I said riders wait for drivers, sometimes it's the other way around. When I showed up, two cars were waiting there. The car at the front of the line already had one passenger, and they were just waiting for a third person (me) to show up. So I climbed in and off we went.
Worked out great for me. Much more comfortable than the bus, and faster too. I'm heading out now to do it again.